The invention relates to a coating apparatus for producing a heat-insulating layer, in particular a heat-insulating-layer system, on a base and to a method of coating a base under vacuum with a heat-insulating layer.
U.S. Pat. No. 5,238,752 discloses a heat-insulating-layer system having an intermediate bond coating. The heat-insulating-layer system is applied to a metallic substrate, in particular a Crxe2x80x94Co steel for an aircraft powerplant blade. Cobalt- or nickel-based alloys are specified as further materials for the substrate. Applied directly to this metallic substrate is an intermediate layer, in particular consisting of a nickel aluminide or a platinum aluminide.
Adjoining this intermediate coating is a thin ceramic layer of aluminum oxide, to which the actual heat-insulating layer, in particular consisting of zirconium oxide stabilized with yttrium oxide, is applied. This ceramic heat-insulating layer of zirconium oxide has a rod-shaped structure, the rod-shaped stems being oriented essentially perpendicularly to the surface of the substrate. This is intended to ensure an improvement in the cyclic thermal loading capacity. The heat-insulating layer is deposited on the substrate by means of an electron-beam PVD (physical vapor deposition) method, in the course of which zirconium oxide and yttrium oxide are evaporated from a metal-oxide body by means of an electron-beam gun. The method is carried out in an apparatus in which the substrate is preheated to a temperature of about 950xc2x0 C. to 1000xc2x0 C. The substrate is rotated in the stream of metal oxide during the coating operation. Details of the rod-shaped grain structure and its properties cannot be gathered from U.S. Pat. No. 5,238,752. The electron-beam PVD method of producing ceramic coatings having a rod-shaped grain structure is also described. In U.S. Pat. No. 5,087,477 and U.S. Pat. No. 5,262,245, the deposition of zirconium oxide on a substrate being effected in an oxygen atmosphere.
Further methods and examples of applying a heat-insulating-layer system to a gas-turbine blade are described in U.S. Pat. No. 5,514,482 and U.S. Pat. No. 4,405,659.
According to U.S. Pat. No. 4,405,659, it is supposed to be possible with the electron-beam PVD method to apply a heat-insulating layer of zirconium oxide which is stabilized with yttrium oxide and has a thickness of about 125 xcexcm and a columnar structure. The average cross-sectional area of a stem is said to be in the order of magnitude of 6.5 xcexcm2.
U.S. Pat. No. 4,321,310 and U.S. Pat. No. 4,321,311 each describe heat-insulating-layer systems which have an adherent layer between the zirconium oxide and the metallic substrate with an alloy of the MCrAlY type. Here, xe2x80x9cMxe2x80x9d stands for one of the metals cobalt, nickel or iron, xe2x80x9cCrxe2x80x9d stands for chromium, xe2x80x9cAlxe2x80x9d stands for aluminum and xe2x80x9cYxe2x80x9d stands for yttrium. A PVD (physical vapor deposition) method is disclosed as a possible method for producing a heat-insulating layer of zirconium oxide.
The coating of metallic components, in particular gas-turbine blades consisting of a superalloy, from a composite system having an adhesive layer and a heat-insulating layer is likewise described in International patent Application WO 93/18199 A1. Here, the heat-insulating layer is preferably applied by the electron-beam PVD method, although other PVD methods, such as sputtering and arc deposition, could also be suitable for this purpose.
In the article xe2x80x9cZirconia thin film deposition on silicon by reactive gas flow sputtering: the influence of low energy particle bombardmentxe2x80x9d by T. Jung and A.Westphal, in Material Science and Engineering, A 140, 191, pages 528 to 533, the so-called reactive gas-flow sputtering method is disclosed for producing zirconium-oxide layers on semiconductor substrates, in particular on a silicon basis. Here, the method relates to the cold deposition of zirconium oxide, which leads to an amorphous growth of the zirconium oxide. This amorphous deposition is effected at substrate temperatures of markedly less than 800xc2x0 C., heating of the substrate being effected directly, with losses by the substrate carrier. For this purpose, the substrate carrier itself can be heated up to at most a temperature of about 800xc2x0 C., so that, with due allowance for the heat losses which occur, heating of the substrate to over 400xc2x0 C. can be achieved.
In German Democratic Republic Patent No. DD 294 511 A5, corresponding to this article, an inert gas, in particular argon, is passed through a hollow cathode, in the interior of which an anode is arranged, so that ionization of argon atoms takes place. The latter strike the cathode, as a result of which cathode material passes into the interior of the hollow cathode and is passed out of the latter with the inert-gas flow. The cathode material is a pure metal, to which oxygen is fed outside the hollow cathode, so that complete oxidation of the metal, in particular zirconium, takes place. Here, the partial pressure of the fed oxygen is in the order of magnitude of 10xe2x88x924 Pa. The total dynamic pressure in the vicinity of the semiconductor to be coated is about 13 Pa to 24 Pa. The deposition rate is about 15 nm/min, the substrate having a temperature of about 400xc2x0 C. The hollow cathode is designed as a cylindrical tube of zirconium having a percentage purity of 99.7%.
An alternative design of the hollow cathode for achieving a larger coating area and a higher coating rate is described in the article xe2x80x9cHigh rated deposition of alumina films by reactive gas flow sputteringxe2x80x9d by T. Jung and A. Westphal, in Surface and Coatings Technology, 59,1993, pages 171 to 176 (corresponding to German patent Application DE 42 35 953 A1). The hollow cathode disclosed is of linear construction in the sense that plates of zirconium are arranged next to one another in a housing. An inert-gas flow can be passed between each two adjacent plates, so that a plasma of inert-gas atoms forms between adjacent plates. In addition, the plates may have a cooling feature, in particular cooling passages. Test bodies of silicon, stainless steel and glass were coated by the hollow cathode and the strength of the aluminum-oxide layer was tested up to around 200xc2x0 C. Nothing is said in the two articles mentioned, concerning the structural properties of the oxide layers with regard to crystallite size and orientation.
It is accordingly an object of the invention to provide a coating apparatus for producing a heat-insulating layer, in particular a heat-insulating-layer system, on a base and to a method of coating a base under vacuum with a heat-insulating layer, that overcome the above-mentioned disadvantages of the prior art components, apparatus, and methods.
With the foregoing and other objects in view there is provided a heat resistant component comprising a substrate and a ceramic heat-insulating layer which is arranged thereon and has a structure having ceramic stems which are oriented mainly normal to the surface of the substrate and have an average stem diameter not exceeding 2.5 xcexcm and preferably 0.5 to 2.0 xcexcm.
There is also provided a coating apparatus for producing a heat-insulating layer on a substrate, comprising a) a holding device for positioning the substrate, b) a hollow-cathode arrangement through which an inert gas can flow and which comprises a cathode material and an anode and has a gas-outlet opening facing the holding device, and a gas-inlet opening for inert gas, and c) a separate heating device for directly heating the substrate with heat radiation and/or convection.
There is moreover provided a method of coating a substrate under vacuum with a heat-insulating layer, in which method an inert gas is ionized in an essentially oxygen-free atmosphere, and the ionized inert gas releases from a metallic cathode material metal atoms, which are carried along with the inert gas in the direction of the substrate and to which oxygen is fed before the substrate is reached, so that a metal oxide forms and is deposited on the substrate, or metal is deposited on the substrate and is oxidized by oxygen coming into contact with it, the substrate being heated to a predetermined germinating temperature of over 800xc2x0 C.
Thermal protection of the substrate, particularly a metallic substrate, is ensured by a ceramic heat-insulating layer. However, known ceramic structures are susceptible to cyclic thermal stresses and can tend to break apart and/or lose their adhesive properties. The resistance to alternating thermal stresses is markedly increased by a microstructure having ceramic stems of smaller diameter than the diameter achievable hitherto. A microstructure having ceramic stems of an average stem diameter of less than 2.5 xcexcm, in particular between 0.5 xcexcm and 2.0 xcexcm, has a high expansion tolerance and a high cyclic loading capacity on account of its orientation essentially perpendicularly to the surface of the substrate and of the columnar fine structure. This readily compensates for differences in the coefficients of thermal expansion of the substrate, particularly a metallic substrate, and the ceramic heat-insulating layer. Such small stem diameters can be achieved by a reactive gas-flow sputtering method developed for high temperatures.
For good bonding of the ceramic heat-insulating layer to the metallic substrate, in particular consisting of a nickel-based alloy or other alloys suitable for producing components which can be subjected to high thermal stress, a metallic adhesive layer is applied to the substrate. With regard to the layer thickness of a few micrometers, reference may be made to U.S. Pat. No. 5,238,752, U.S. Pat. No. 4,321,310 and U.S. Pat. No. 4,321,311. In accordance with an additional feature of the invention, the metallic adhesive layer preferably consists of an alloy of the McrAlY type, where M stands for one or more of the elements iron, cobalt or nickel, Cr stands for chromium, Al stands for aluminum and Y stands for yttrium or one of the elements of the rare earths. An intermetallic compound, e.g. consisting of nickel aluminide or platinum aluminide, is likewise suitable as the adhesive layer.
In accordance with another feature of this invention, it is also advantageous, in particular with regard to prolongation of the service life and adhesion of the heat-insulating layer to the substrate, to produce a chemical bond between the heat-insulating layer and the metallic adhesive layer. This is achieved, for example, by a thin layer of an inorganic aluminum compound such as aluminum oxide (Al2O3). A layer of a ternary Alxe2x80x94Zrxe2x80x94O compound or an Alxe2x80x94Oxe2x80x94N compound is likewise suitable as mediator layer. The ternary Alxe2x80x94Zrxe2x80x94O compound, e.g. Al2Zr2O7, is preferably suitable for the bonding of a heat-insulating layer which contains zirconium oxide. In the case of other ceramic heat-insulating layers, for example consisting of magnesium oxide, other spinels may accordingly be used. A layer of aluminum nitride or a compound (mixed layer) of aluminum nitride and aluminum oxide is also suitable.
In a further feature according to this invention, the heat-insulating layer preferably includes a metal-ceramic substance, in particular zirconium oxide (ZrO2). This metal oxide is preferably produced with a stabilizer, such as yttrium oxide (Y2O3), in order to avoid a phase transformation at high temperatures. A content of 3% by weight to 12% by weight, in particular 7% by weight, of yttrium oxide is preferably added to the zirconium oxide.
A ceramic heat-insulating layer having a fine columnar structure with an average stem diameter of less than 2.5 xcexcm is especially suitable for thermal protection of components of a gas turbine which are subjected to cyclic thermal stress of more than 1000xc2x0 C. These include in particular gas-turbine blades as well as components of a heat shield of a combustion chamber of a gas turbine. This applies both to stationary gas turbines for use in power stations and to aircraft powerplant turbines. Of course, the heat-insulating layer according to the invention is also suitable for other components subjected to high thermal stress.
The coating apparatus for producing a heat-insulating layer on a substrate according to this invention has a holding device for positioning the parent body, a hollow cathode through which an inert gas can flow and which comprises a cathode material and an anode and has a gas-outlet opening, facing the holding device, and a gas-inlet opening for inert gas, and an additional, separate heating device for heating the substrate.
In the hollow-cathode arrangement, which is of hollow-cylindrical design with a circular or a rectangular cross-section, a glow discharge is produced by applying a direct-current voltage between cathode and anode.
The anode may be designed, for example, in a rod shape and may be arranged within the cathode or outside the cathode, in particular so as to enclose the cathode as a housing. Due to a plasma which develops in the cathode, such a large voltage drop is present in each case between the plasma and the cathode that constant ionization is maintained. Inert gas entering the gas-inlet opening is thereby ionized inside the hollow cathode. The ionized inert-gas atoms strike the cathodically connected metal surfaces of the hollow cathodesxe2x80x94this being an ion bombardment so to speakxe2x80x94and lead to an at least partial atomization of the metal surfaces. The cathode material preferably consists of an alloy of zirconium, as a result of which zirconium atoms or zirconium-atom clusters are driven out of the cathode material by the ion bombardment. In accordance with the feature of desired stabilization of the subsequent oxidized zirconium deposited on the substrate according to this invention, a stabilizing metal, such as yttrium, is admixed or alloyed with the cathode material. Accordingly, the cathode material has a predetermined area ratio or volumetric ratio of metallic zirconium and yttrium.
For the oxidation of the zirconium and, if need be, the admixed yttrium in accordance with this invention, an oxidizing-medium feed is provided in the apparatus outside the hollow cathode, through which oxidizing-medium feed oxygen in particular can be fed in appropriate quantities. The atomized zirconium particles and yttrium particles, which are present as metal atoms and/or metal clusters, are transported out of the hollow-cathode arrangement with the gas flow of the inert gas, in particular argon. These particles are completely oxidized outside the hollow cathode in a controlled oxygen reaction atmosphere. This is done by feeding oxygen through the oxidizing-medium feed in such a way that, in combination with the inert-gas flow, ingress of oxygen into the hollow cathode is prevented, so that oxidation of the cathode material is largely avoided. The oxidized metal particles are deposited on the substrate as a metal-oxide-ceramic heat-insulating layer. The oxidation may also take place immediately after the deposition on the surface of the substrate. To achieve the desired columnar structure, the pressure inside the coating apparatus as well as the temperature, in particular the temperature of the substrate, are controlled appropriately. The substrate is heated via a heating device to a temperature of over 900xc2x0 C., in particular to 950xc2x0 C. and up to about 1050xc2x0 C.
With the coating apparatus according to this invention for carrying out a high-temperature gas-flow sputtering method, decoupling of the working atmosphere for the ionization of the inert gas (plasma source) from the component to be coated is achieved. Compared with conventional PVD methods, in particular the electron-beam PVD method, there are different value ranges for the characteristic variables, such as pressure of the residual gas (reduced pressure, requisite pumping level of the vacuum system), working pressure and ratio of the reactive gas (oxygen) to the remaining working gases. The atmosphere at the component to be coated can be at a residual-gas pressure of 10xe2x88x923 mbar for carrying out the method, in which case the upper limit of the residual pressure can be in the order of magnitude of 10xe2x88x922 mbar. The working pressure at the component (main chamber of the coating apparatus) can be in the order of magnitude of between 0.2 and 0.9 mbar. The ratio of the reactive gas (oxygen) to the ionized inert gas (plasma gas), e.g. argon, can be in the range of 0.01 to 0.04.
The atmosphere of the plasma source is essentially isolated from the atmosphere of the component and has a residual-gas pressure in the order of magnitude of the residual-gas pressure at the component. The working pressure of the gas flow can be about 0.02 mbar higher than the working pressure at the component. The working pressure at the component, i.e. in the main chamber of the coating system, is therefore determined by the gas outflow of the source. In the hollow cathode, therefore, there is a positive pressure relative to the main chamber. The ratio of residual-gas constituents, in particular oxygen, to the ionized inert gas (plasma gas) is preferably less than 1%. In this way, the ionization source (the hollow-cathode arrangement) can be run by direct-current operation, since no oxidation of the coating source (hollow cathode) is effected with an unsteady operating state. The occurrence of a glow discharge as well as the production of arc plasma on account of oxidation of the cathode material are thereby avoided.
Compared with known apparatus for carrying out PVD methods, in particular the electron-beam PVD method, a markedly higher residual-gas pressure is thus possible, which leads to a simplified and more cost-effective vacuum system. The residual-gas pressure in the known systems for achieving a columnar heat-insulating layer is in the range of 10xe2x88x926 to 10xe2x88x925 mbar. In the conventional methods, the working pressure is within the range of 10xe2x88x923 to 10xe2x88x922 mbar with a technically useful upper limit of less than 0.1 mbar and is thus markedly below the working pressure possible in the reactive gas-flow sputtering method. In addition, a high ratio of reactive gas (oxygen) to other working gases, such as argon, helium, etc., of 10:1 or higher is necessary in the known PVD methods. In the apparatus according to the invention and the method according to the invention, a substantially lower ratio is required, so that the devices for feeding working gas and reactive gas can also be of markedly simpler and less expensive design. In addition, in the known PVD methods, the coating source is not isolated in the main chamber and therefore also lies exposed to oxidation. The apparatus according to the invention can therefore be designed without high-frequency generators or real-time-controlled direct-current generators.
Here, the heating device is preferably designed in such a way that there is uniform heating, in particular volumetric heating, of the substrate. Even at points of the substrate having high mass concentration and large partial volume, a uniform germinating temperature for the substrate as a whole is achieved.
The vacuum (working pressure) inside the coating apparatus is preferably less than 1 mbar and in particular is within the range of between 0.3 mbar and 0.9 mbar, such as, for example, around 0.5 mbar. To establish the desired vacuum, a vacuum pumping device is provided, and this can have a simple construction, e.g. a Roots pump design. Compared with the conventional electron-beam PVD method, in which a rotary-vane backing pump as well as a diffusion pump are to be provided in order to achieve a high vacuum, the vacuum pumping device of the gas-flow sputtering method can be of substantially simpler design.
To achieve as uniform a coating of the component, in particular a gas-turbine blade, as possible, the holding device is adapted for motion of the substrate relative to the gas-outlet opening. The holding device preferably contains a turning mechanism, by means of which continuous rotation of the component about its longitudinal axis is effected.
The object directed towards a method of coating a substrate under vacuum with a heat-insulating layer is achieved by an inert gas being ionized in an essentially oxygen-free atmosphere. This is effected, for example, by the inert gas being passed through a hollow cathode and ionized in the latter. The ionized atoms of the inert gas release from a metallic cathode material metal atoms and/or metal clusters, which are passed out of the hollow cathode with the inert gas and are oxidized with oxygen outside the hollow cathode to form a metal oxide. It is likewise possible for metal to be deposited on the substrate and oxidized there by oxygen coming into contact with it. The metal oxide is deposited on the substrate, which is heated by a separate heating device to a predetermined germinating and condensation temperature. A heat-insulating layer consisting of a metal oxide is thereby produced on the substrate, which heat-insulating layer has a fine columnar microstructure, in which case the average stem diameter can be less than about 2.5 xcexcm and in particular can be within a range of between 0.5 xcexcm and 2.0 xcexcm. This heat-insulating layer has especially good resistance to thermomechanical alternating stresses, as is especially advantageous in particular in the case of parts of a gas-turbine plant, such as turbine blades and insulating components, exposed to hot gas.
In contrast to known electron-beam PVD methods, a pure metal or an alloy consisting of a principal metal and at least one stabilizing metal is used as the cathode material according to this invention. An alloy of zirconium with yttrium is particularly suitable for this, the yttrium being added to the zirconium in such a quantity and distribution that a heat-insulating layer of zirconium oxide partly stabilized with yttrium oxide is obtained.
Other metals which lead to a thermally highly resistant metal oxide, such as magnesium for example, are of course also suitable as cathode material.
The use of a metallic cathode according to this invention instead of a body of metal oxide, as is used, for example, in the known electron-beam PVD methods, has the advantage that the heat-insulating layer produced has a substantially finer structure. Furthermore, the appearance of layer defects, which may occur in the electron-beam PVD method due to defects in the ceramic body, such as inhomogeneous porosity or the effects of impurities, is avoided by a fully reactive oxidation process of the metallic atomization materials released from the cathode material. In addition, the cathode material can be produced in a simpler manner compared with a ceramic body and with extremely high purity.
The bonding of the heat-insulating layer of metal oxide is effected, for example, via the formation of a homogeneous, growing aluminum oxide reaction layer (mediator layer) between the heat-insulating layer and an adhesive layer consisting of a metal alloy of the MCrAlY type. In addition, the reactive gas-flow sputtering method, with the use of a hollow-cathode arrangement through which an inert gas flows, has the advantage that it can be carried out in a relatively low vacuum with sufficient deposition of metal oxides on the substrate. Compared with the known electron-beam PVD methods with complicated electron-beam deflecting and focusing functions, the method described is distinguished by simple control or regulation of the process variables, such as germinating temperature, level of vacuum, oxygen partial pressure, volumetric flow of the inert gas, and output of the hollow-cathode discharge. The process variables required to achieve a structure having an average stem diameter of less than 2.5 xcexcm are determined with the aid of the Thornton diagram for the formation of the PVD layer structure, as described by J. A. Thornton, for example, in the Journal of Vacuum Science Technology, vol. 11, 1974, pages 666 to 670. Described therein is the formation of the layer structure as a function of the substrate temperature, the vacuum-chamber gas pressure and the coating energy content for activating plasma-exchange processes.
The anode does not wear out, since it is not subjected to any coating or oxidation when arranged in the gas-inlet region. Wearing parts, such as anode and cathode, can be kept small, in particular since the anode is arranged inside the hollow cathode and is therefore not subjected to a direct bombardment of electrons or ions. In addition, the anode can be produced with high degree of purity. Furthermore, it is advantageous that the materials to be atomized themselves function as cathode and do not have to be supplied as metal-oxide bodies with predetermined mixture ratio.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a coating apparatus for producing a heat-insulating layer, in particular a heat-insulating-layer system, on a base and to a method of coating a base under vacuum with a heat-insulating layer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.